1. Field of the Invention
The present invention relates to the preparation of bisphenol A by a condensation reaction of phenol and acetone, and, more especially, to the preparation of bisphenol A by condensing phenol with acetone in the presence of an acid catalyst and a particular polymer additive or co-catalyst.
2. Description of the Prior Art
Bisphenol A or 2,2-bis(4-hydroxyphenyl)-propane is a valuable intermediate known to be useful in the production of epoxy and polycarbonate resins.
Numerous processes for the production of bisphenol A are known to this art. The most widely known comprises the reaction of acetone with an excess of phenol in the presence of an acid catalyst at temperatures on the order of 30.degree. to 80.degree. C.
The catalyst can be a solid or liquid inorganic acid, such as a clay, aluminum trichloride, hydrochloric acid or sulfuric acid, or an organic acid such as benzenesulfonic acid (cf. published Japanese Application 62-5926 and U.S. Pat. No. 4,387,251), or an ion exchange resin (c. AIChEJ, 20 (5), pp 933-40 (1974)).
The processes using heterogenous catalysts present the major disadvantages of necessitating operation at a low bisphenol A concentration, such as to prevent the precipitation of the complex (bisphenol A/phenol), or at higher temperatures, which promote the formation of by-products. It is for these reasons that, more preferably, an acid soluble in the reaction mixture is used, such as hydrochloric acid. Hydrochloric acid, in turn, presents the major disadvantage of necessitating reactors which are made of materials capable of resisting extensive corrosion; therefore, they are very expensive. As the majority of the other acids are less active than hydrochloric acid, the contact times must be increased to obtain an acceptable or complete degree of conversion of the acetone. However, such an increase in the contact time is reflected in the appearance of particularly undesirable parasitic species such as 2-(o-hydroxyphenyl)-2-(p-hydroxyphenyl)propane (o,p'isomer of bisphenol A), 2,2,4-trimethyl-4-(p-hydroxyphenyl)chromane or codimer, and higher condensation products such as trisphenol. These species can be separated from the bisphenol A only by a succession cf purification steps which are often difficult to carry out and are always relatively costly.
Moreover, for certain applications, it is imperative that the bisphenol A has no coloration whatsoever and has a very high degree of purity. This is the case, in particular, when using the bisphenol A for the synthesis of polycarbonate resins, in which the presence of foreign substances has a pronounced inhibitory effect.
In this regard, it has already been proposed to this art to add a "co-catalyst" or additive to the (phenol/-acetone/acid catalyst) system.
The co-catalyst is most typically a sulfur compound such as an alkyl mercaptan (cf. Netherlands Patent Application No. 73/09,229), an alkylmercaptocarboxylic or -sulfonic acid (cf. Australian Patent No. 474,155 and French Patent No. 1,179,377) or an amino alkanethiol (cf. Japanese Patent Application No. 74/20,565).
However, the use of these accelerators does not prove fully advantageous: they still present serious difficulties in treatment for their removal and/or their recovery and they often impart an undesired odor to the final product.
The use of another class of "co-catalysts" has been described in U.S. Pat. No. 4,052,466, i.e., poly(hydroxy)benzene derivatives, in particular resorcinol and the monomethyl or dimethyl ethers thereof, which are used in an amount of 0.1% to 10 mole % relative to the phenol employed. These derivatives provide results similar to those obtained with, for example, the mercaptans. Indeed, they activate the phenol/acetone condensation reaction in the presence of an acid catalyst and tend to reduce the parasitic chemistry.
However, these derivatives reflect the major drawback of pursuing the same route as the organic impurities of bisphenol A in the production chain. They have an adverse influence on the crystallization of the bisphenol A/phenol adduct and remain present, at least in trace amounts, in the final product.
In such context, it will be appreciated that serious need exists in this art for additives which are capable of activating the condensation reaction of phenol and acetone without promoting parasitic competing reactions and which proceed by a mechanism different from that of the impurities and/or bisphenol A in the production chain.